It is recognized that many solid high temperature materials produced in the form of elongated crystalline bodies exhibit properties which, depending upon the configuration of such bodies, render them useful for a wide variety of applications. Elongated single crystal or essentially monocrystalline bodies, in particular filaments, of selected materials such as .alpha.-alumina have utility as reinforcement elements for metal, intermetallic and ceramic matrices to provide composite materials useful in fabricating structural parts for jet engines, turbines, etc., as substrates for epitaxially grown integrated circuit devices and as optical components.
Heretofore, as taught in a series of patents issued to LaBelle and others, the elongated crystalline filaments are pulled from a melt by a process called "edge defined film fed growth" (EFG). Briefly described the process involves growth of a selected material in the form of an elongated body with an arbitrary, constant cross-section from a shaping member (or die) having a substantially horizontal surface whose gross geometry with respect to its bounding edges is the same as that of the elongated body to be produced. The shaping member has one or more orifices through which material in a molten state is fed to form on the aforesaid surface a film having the same geometry as the surface. The elongated body is grown from the film by causing the molten material to solidify on a seed body which is withdrawn in a substantially vertical direction at a speed consistent with the rate at which the molten material can be supplied to maintain the film and/or the rate at which liberated heat of solidification can be rejected. The shaping member is made from a material which is compatible with the liquid and solid phases of the material to be grown and which is wetted by the material. The shaping member is disposed in a reservoir supply of the material in the molten state and each orifice is dimensioned so that the surface tension forces are sufficient to cause continuous feeding of the material from the reservoir supply by capillarity. By proper adjustment of the melt temperature and the rate of seed withdrawal, a crystalline body having the same cross-sectional geometry as the surface of the shaping member can be produced on a continuous basis so long as the liquid material continues to be supplied at a rate sufficient to maintain the film between the shaping member and the growing body and sustain the desired growth rate.
However, for small diameter filaments useful for metal matrix composites, e.g. titanium matrix, YAG fiber composites and especially for very small diameter filaments useful for ceramic matrix composites, e.g. alumina matrix, alumina fiber, the fabrication of the die used in EFG becomes very difficult.
In EFG crystal growth the die capillary is filled by capillary action and upon seeding, the melt spreads out over the top surface of the die and the diameter of the grown crystal filament is controlled by the outside diameter of the top surface of the die. The capillary and top surface are formed of wetting materials. (Boatman, a United States Patent, teaches an entirely non-wetting die). With very small filament dies, the top surface of the die tends to be very small, approaching an edge. In this situation, however, the die (tungsten or molybdenum) is so thin that seepage of the crystalline melt, e.g. sapphire, through the grain boundaries tends to produce spilling and wetting down the outer surfaces of the side walls of the die tip. This produces a larger diameter filament than desired and also an undesired solid--liquid interface shape.
Accordingly, an object of this invention is to provide a new method and system by which solid crystalline materials can be pulled from a melt as elongated crystalline bodies of indefinite lengths and predetermined cross-sectional configurations especially materials of small cross-section such as filaments.
A further object is to provide a method and a system whereby elongated bodies of solid materials may be grown from a melt to various arbitrary shapes and sizes with smooth surfaces.
Another still further object of this invention is to provide a method and a system by which congruently melting solid materials may be continuously pulled from a melt in the form of crystalline bodies of controlled cross-sectional configurations.
A still further object of this invention is to provide a novel method and a system for growing from the melt elongated single crystal, essentially monocrystalline, and polycrystalline bodies of indefinite length and predetermined cross-sectional configuration.
A still further object is to produce elongated crystalline bodies of the character described by a growth process in which the melt temperature and pulling speeds may be varied over relatively wide limits without effecting any substantial change in product cross-section.
A specific object of this invention is to grow from melts of selected material extended crystalline bodies characterized by a variety of predetermined cross-sectional configurations, including but not limited to round filaments, flat ribbons, round hollow tubes, etc.
The foregoing and other objects and advantages of the invention are embodied by what is hereinafter referred to as an "inside edge defined, self-filling" (IESF) die, the method of using the die and the system in which the die is embodied.
Broadly, the invention comprises a die tip, the top and outer surfaces being characterized by a non-wetting material, such as boron nitride or the carbides of molybdenum or tungsten. This die tip achieves a major improvement in crystal growth particularly for small diameter filaments.
With the invention, the die is still filled to the top by capillary action as in EFG.
However, the diameter of the filament is controlled by the inner surface of the non-wetting material which is typically the inside diameter of the die tip. Converting the outside of the die walls to the non-wetting composition stops the intergranular seepages or leaking and wetting of the die.
Die fabrication is much more practical.
a. In EFG, a die used to produce 0.003" diameter filament would have an OD of about 0.003" and an ID of about 0.001". PA1 b. This EFG die is a very fragile and difficult to fabricate structure. PA1 c. Even if adequately made, the top edge of the EFG die would tend to be rounded and lead to a wide variety of diameter filaments grown from the die top. In other words, diameter control would not be particularly good. PA1 d. The IESF die for the same 0.003" diameter filament could be 0.006" OD and 0.003" ID. PA1 e. The IESF die is a comparably robust structure and much easier to produce. PA1 f. The IESF die top can be flat and rounding of the outside surface edge is of no consequence. In fact the outer wall can be tapered to produce even more strength without promoting spilling or wetting of the liquid crystalline down the outer surface. PA1 g. Diameter control of the IESF die is greatly increased with little variation of the filament diameter with variations of growth parameters.
Broadly the invention comprises a die used in combination with an apparatus for growing crystalline filament. The die is in communication with a capillary through which rises a crystalline melt. The die has an upper surface having an aperture through which the melt is drawn. The aperture is defined by an inner edge. The upper surface of the die tip is non-wetting with reference to the melt. The geometry of the melt drawn through the die is defined by the inner edge.